A number of arrangements are known in the art for coupling signals from a bidirectional transmission facility to receive and transmit unidirectional transmission facilities. One example of this type transmission network is employed to couple a two-wire bidirectional telephone transmission facility to a four-wire transmission facility. These arrangements are commonly referred to as hybrid circuits, whether they employ a hybrid transformer or not.
As is now known, it is desirable to adjust a complex impedance circuit in the transmission coupling network in order to obtain a better match to the impedance of the bidirectional transmission facility thereby maximizing loss between the receive and transmit unidirectional facilities. This is commonly referred to as maximizing transhybrid loss.
Many transmission networks employ so-called hybrid networks to realize the desired two-to-four wire coupling. Transmission networks employing either hybrid transformers or electronic circuits are now commonly employed in telephone transmission systems to realize the desired two-to-four wire coupling. In using either a hybrid transformer or an electronic "hybrid" it is desirable to employ a network having an impedance which substantially matches the impedance of the bidirectional transmission facility. Otherwise, low transhybrid loss results which, in turn, yields unwanted signal reflections. That is to say, a portion of the signal on the receive unidirectional facility appears in the transmit unidirectional facility. To this end, in transformer type hybrids, a complex impedance network is employed in an attempt at matching the impedance of the two-wire facility. Similarly, in electronic canceller type hybrids, a network having a complex transfer (impedance) characteristic is employed to generate an error signal in attempting to cancel unwanted signals in the transmit unidirectional facility.
In either arrangement adjustable impedance networks have been used in order to obtain a better impedance match to the bidirectional facility and, hence, to maximize transhybrid loss.
In transmission networks which employ hybrid transformers it has become the practice to employ an electronic network to generate a driving point impedance which emulates the complex impedance of the bidirectional transmission facility and, thereby, balance the hybrid transformer. Heretofore, the balance network was manually adjusted in an attempt at obtaining an optimum match to the impedance of the bidirectional facility. In one known arrangement, a random noise signal is applied to the receive port of the hybrid transformer while elements of the balance network are manually adjusted to obtain amplitude null indications at the hybrid transmit port. Three separate noise bands have been employed in an attempt at manually obtaining optimum adjustment, for example, for a non-loaded two-wire facility. Manual adjustment of balance networks is undesirable because of cost factors and the time required to make the adjustment. Indeed, in such arrangements it is practical only to make a manual adjustment upon installation. Therefore, any change in the impedance of the two-wire facility because of either a change in the length thereof or otherwise requires another manual adjustment. Reliance on human adjustment is also undesirable because of possible errors.
More recently, balance of hybrid transformers in coupling networks has been achieved automatically by employing an adjustable balance network in conjunction with a control circuit. One prior automatic balance network is disclosed in U.S. Pat. No. 4,096,362 issued to C. D. Crawford on June 20, 1978. In the Crawford balance network, a control circuit is employed which includes analog circuits responsive to signals developed at ports of the hybrid to which the bidirectional transmission facility and the balance network are connected. A random noise signal is supplied to the receive port while the transmit port is terminated in a prescribed impedance in an attempt at emulating in service hybrid operating conditions. To this end, the receive and transmit ports of the hybrid are connected to a random noise source and an open circuit, respectively, to obtain the desired circuit arrangement. Individual analog circuits are employed to continuously generate nulling signals for adjusting the impedance elements of an adjustable balance network as disclosed in U.S. Pat. No. 3,919,502 issued to G. T. Daryanani on July 31, 1975. Use of analog circuits in conjunction with a random noise source to obtain the desired nulling signals is undesirable because of the time required to achieve the desired balance settings. Additionally, use of a random noise source to generate signals for developing the null settings is undesirable because of the settling time of the circuit. Moreover, use of the signals developed at the bidirectional facility connection and the balance network connection to the hybrid to generate the nulling control signals causes problems because of noise developed on the bidirectional transmission facility. Consequently, bandpass filters are required to limit the input to the control signal generating circuit. In turn, this increases response time of the control circuit. Furthermore, employing signals detected at the bidirectional facility connection and the balance network connection to the hybrid transformer also requires transformer coupling to achieve desired isolation of the circuit functions. Therefore, this prior balance arrangement is not readily fabricated by utilizing large-scale integration. Furthermore, the slow response of prior analog circuit arrangements is undesirable when employing the coupling circuit in switched telephone systems.
Thus, although the prior known balance arrangements are satisfactory for some applications they are undesirable for others where rapid response time is required; for example, when it is desired to achieve a new balance for each telephone off-hook condition.